1. Field of the Invention
The present invention is directed to combustible gas analysis apparent. More specifically, the present invention is directed to a gas analysis apparatus for determining the heating Wobbe Index of combustible gases.
2. Description of the Prior Art
The well-known Wobbe Index of a combustible gas is defined as the amount of heat released by a burner of constant orifice, McGraw-Hill Dictionary of Science, 1979 and stated mathematically by the following relation:
W=H/.sqroot.SG where H is the caloric combustion value of the gas per unit volume, e.g., BTU/ft.sup.3, and SG is specific gravity of the combustible gas. The Wobbe Index is a quantity used in heating technology since different combinations of gases supplied to a gas heated apparatus under the same pressure provide equal heat production from the apparatus whereby the apparatus does not need to be readjusted as long as the Wobbe Index is maintained at a predetermined value. For example, if a mixture of gases from different sources is burned in an industrial heating operation, the gases must be mixed in such proportion so that a gas is obtained having a constant Wobbe Index. One prior art method of determining the Wobbe Index of the gas involves the combination of a caloriometer, a density meter and a computing circuit, e.g., a microprocessor. These parts while they may be combined into a single instrument produce an overall device which is very costly and exhibits a sluggish operation whereby rapid changes in the gas mixture are measured at a slow rate and the resulting correction, if necessary, is also applied at a corresponding slow rate. It has been discovered that when gas mixtures having different compositions and different Wobbe indices are burned with equal quantities of air, the oxygen content of the exhaust gas shows a direct correlation with the Wobbe Index. Accordingly, for purposes of measurement and control, it is not necessary to measure the Wobbe Index as such and it is sufficient to measure only the oxygen content in the exhaust gas. One prior art apparatus for producing this type of operation includes a sampling line containing a flow control nozzle for withdrawing a gas sample, a means for adjusting the gas sample so that the pressure difference through this control nozzle has an adjustable constant value, a means for feeding a constant volume of air as a source of combustion oxygen into the gas stream sample, a combustion chamber, a burner in the combustion chamber to completely burn the gas-air mixture, an outlet for the burned gas from the combustion chamber and an oxygen sensor in the combustion chamber to sense the oxygen content of the exhaust gas. This oxygen content is a quantity which is correlated to the Wobbe Index of the measured gas. However, by actually measuring or providing a quantity which represents the Wobbe Index the thermal delivery rate, i.e., BTU/minute could be obtained to control the industrial heating operation. Thus, the measuring apparatus providing such a Wobbe Index measurement would provide an efficient means of "feed forward" control to a furnace requiring a constant BTU per minute input without regard to changes in gas supply composition, density and BTU content.